Irradiation of vinyl chloride polymer plus two unsaturated monomers



United States 9 Claims. or. 204-154 This invention relates toimprovements in vinyl chloride polymer compositions.

Recent advances in the field of radiation chemistry and atomic energyhave shown the need for materials which may be used as wire insulation,tubing, gaskets, etc., in equipment which is located in areas of highradiation flux, such as near X-ray apparatus, atomic reactors, etc., andwhich will not become discolored by exposure to such radiation, andwhich will not lose desirable physical properties when exposed to suchradiation. It is the aim of this invention to produce such improvedmaterials.

Accordingly, the present invention comprises introducing a monomericester of an alpha-unsaturated dicarboxylic acid and monomeric vinylidenecompound into a vinyl chloride polymer.

All varieties and grades of resinous polymers consisting of polymerizedvinyl chloride or of polymerized vinyl chloride and minor quantities ofcopolymerizable vinyl monomers may be used in the practice of thisinvention. The choice will depend upon the processing conditionsemployed and the properties desired in the finished product. Mixtures oftwo or more vinyl chloride polymers or copolymers may also be used. I

A large number of esters of alpha-unsaturated dicarboxylic acids areapplicable to this invention. The alcohol radicals of such esters mayconsist of branched or normal alkyl radicals having one to eighteencarbon atoms. Among the suitable materials are the alkyl esters of thefollowing alpha-unsaturated dicarboxylic acids: maleic acid, fumaricacid, itaconic acid, citraconic acid, mesaconic acid, and methylenemalonic acid. At least one ester of such acids is used in thisinvention. The preferred material is di-Z-ethylhexyl maleate.

The preferred difierent third ingredient of this invention is styrene.Other suitable monomeric vinylidene compounds includealpha-methylstyrene, divinylbenzene, vinyltoluene, monochlorostyrene,dichlorostyrene, and vinylnaphthalene.

The compositions of this invention are useful in two forms. The firstform, in which the monomeric ester and, for example, monomeric styreneare added to and mixed with the polymerized vinyl chloride, with nofurther treatment, finds use where the maximum of resistance to theeffects of ionizing radiation must be obtained. In this instance, thetwo monomers have not combined with the polymerized vinyl chloride, butare merely absorbed in the polymer and act in effect to plasticize thepolymer. Subsequent exposure to ionizing radiation, either as a resultof its exposure during use, or intentional exposure for the purpose ofmodification, results in internal modification of the mixture. The threeingredients are bound together by cross-linkages produced by the actionof the ionizing radiation on the mixture. This second, cross-linked,form of the material of this invention shows increased hardness,increased modulus, increased tensile strength, and increased resistanceto solvents. At the same time, there is only a limited amount ofdiscoloration resulting from the exposure to ionizing radiation.

The first form of the material is usable where producatent tion offinished articles possessing great resistance to degradation ordiscoloration from ionizing radiation is required. A limitation on itsuse in this form lies in the fact that the styrene is not chemicallycombined, and there is as a result, an odor of styrene to the material.In this form, the material is relatively soft and moldable. It istherefore possible to mold or otherwise shape the material into thedesired useful form, and modify the soft material with ionizingradiation into the more rigid second form which will retain permanentlythe shape given to it. This method of fabricating finished articles isof particular interest when the vinyl chloride polymer used is acopolymer of 96-85% vinyl chloride and 4-15 vinyl acetate. When amixture of this copolymer, di-Z- ethylhexyl maleate, and styrene isexposed to ionizing ra-.

diation, it undergoes an unexpectedly large increase in hardness andmodulus. By this method, it is possible to produce hard and rigid endproducts from raw materials which are easily handled by methods used inthe plastics industry, i.e., molding, extrusion, slush-casting, etc.

The second, cross-linked form of the material has a more generalutility, since it possesses the advantage of the first form, namely,resistance to discoloration on exposure to ionizing radiation, and italso has the increased physical properties obtained through thecross-linking step. Further, the styrene component is chemicallycombined in the material, making it useful in applications where thefirst form could not be used due to to the presence of the styrene odor.

The improvements resulting from cross-linking the materials are found inthe increased high temperature tensile strength, and the increasedresistance to solvents.

Ionizing radiation suitable for performing this crosslinking operationis radiation which, when absorbed by matter, produces ionization.Examples include X-rays, gamma-rays, high speed electrons, high speedcharged particles (protons, alpha particles, etc.), and high speedneutrons. electrons are particularly suitable. In the work on thisinvention, exposure to ionizing radiation was accomplished by passingmaterials to be tested through the beam of high speed electrons (2million volts) obtained from a Van de Graaff accelerator, and also byplacing materials in an X-ray field.

Mixing of the ingredients of the material of this invention may beaccomplished in many ways. The method found most convenient was tosimply stir the ingredients together in a Hobart mixer. Other meanscould be employed such as a dough mixer, Banbury, or other similardevice. Open mill mixing is not suitable when styrene is one of theingredients, since this monomer is too volatile at the temperature atwhich vinyl chloride polymers are mixed. However, if substituted styrenemonomers, such as those listed above which are of lower volatility, areused, open mill mixing will be suitable. The mixture should be free ofperoxidic catalysts; they cause polymerization of the vinylidene monomersuch as styrene and thus render the styrene ineffective in preventingdiscolora= tion and promoting cross-linking by ionizing radiation.

The material of this invention may be fabricated in any Way of the Waysknown to the art. Transfer molding, extruding, casting, dipping, orother means of fabrication applicable to plasticized vinyl chloridecompositions may be used. In the examples cited below, molding betweenheated platens of a hydraulic press are used.

The relative amounts, by weight, of vinyl chloride polymer, ester ofalpha-unsaturated dicarboxylic acid, and styrene or the like may beadjusted according to the hardness and stiffness desired in the finalcross-linked product. The total parts of the other ingredients to 100parts of vinyl chloride polymer may range from 25 to 100, that is,

Fatented Oct. 30, 1962..

Of these, X-rays, gamma-rays, and high speed 20-50% based on the totalweight, and the molal ratio of styrene or the like to ester ofalpha-unsaturated dicarboxylic acid may range from 0.5 to 3.0. Thesemolal ratios correspond to the weight ratios of 0.35 to 2 in the case ofstyrene with dimethyl maleate and to weight ratios of 0.085 to 0.5 inthe case of styrene with dioctodecyl maleate. The preferred compositionis composed of 100 parts of vinyl chloride polymer wit-h 50 parts of themixture of the other two ingredients, such mixture being composed of 3parts of di-Z-ethylhexyl malcate and 2 parts of styrene.

With respect to the amount of ionizing radiation to be used inconverting to the cross-linked form of the composition of thisinvention, the lower limit is determined by that amount which isrequired to make the first noticeable change in physical properties, asevidence of crosslinkages being formed. Such evidence of cross-linking,or of chemical combination of the other two ingredients with the vinylchloride polymer, may best be found by examining the total combinedchlorine in the resulting polymer after extraction of uncombinedmaterial. As cross-linking takes place, the percentage of total combinedchlorine will decrease, due to the incorporation of nonchlorinecontaining compounds. A substantial decrease in combined chlorine hasbeen found to occur as a result of a radiation dose corresponding to theabsorption of 4 watt hours of energy per pound of material. This hasbeen selected as the lower limit of radiation required. At dosages ofless than 4 watt hours/pound, cross-linking is taking place, but to alimited extent and having very little effect on the physical propertiesof the material. The upper limit of radiation dose can be determined bythe amount of discoloration taking place upon exposure to ionizingradiation. Measurement of relative light transmission values wasselected as the method of measurement of the amount of discoloration. Bythis method it was determined that the first appreciable changes incolor occur at a dosage of 40 watt hours/pound. In comparison,conventionally formulated vinyl chloride polymers show a steady increasein discoloration, until at 40 watt hours/ pound, the compositions arenearly opaque, and show severe discoloration. The amount of ionizingradiation to be used in the practice of this invention is therefore setat 4-40 watt hours/pound.

Other ingredients such as fillers, plasticizers, coloring agents, etc.,may be added to the polymers of this invention as in the polymers of theprior art. In the claims appended hereto, the term comprisingessentially is intended to mean that the composition consists solely ofmaterials recited therein except for the addition of such inertmaterials not essential thereto such as fillers, coloring agents,plasticizers and the like. This definition is intended to specificallyexclude the presence of any and all peroxide polymerizationcatalysts aswell as monomers and polymers not of' the classes set forth therein.

EXAMPLE 1 The example illustrates the advantages obtained when using thematerials as given in this invention for the compounding of the vinylchloride polymer, as compared to conventionally formulated vinylchloride polymers.

All ingredients were thoroughly blended in a Hobart" mixer, and thenmolded into pieces suitable for testing (6.5" x 6.5" x 0.1") in a platenpress at a temperature of 320 F. These test pieces were then irradiatedwith high speed electrons (2 million volts) from a Van de Graatfaccelerator. The dose, or amount of energy supplied to the material, wascontrolled by variation of the time the material was allowed to remainin the electron beam, and has been estimated in watt hours of energyreceived per pound of material.

The irradiated materials were then tested by standard methods of testingrubbers and plastics, with results as listed in Table I below.

T ionizing radiation on an unprotected composition.

4 T able I Polyvinyl chloride (parts) 100 100 100 Dioctyl phthalate(parts) 50 30 Di-2ethylhexyl maleate (parts) 50 30 Styrene (parts) 20 20Properties Before Irradiation:

Relative light transmission,

percent 77 75 00 84 Combined chlorine, percent 55. 1 5 G 55.3 55. 2Solubility in THE sol. sol. sol. sol. Tonsil strength at 72 F. (p.s.i.)2, 620 2, 870 2,340 2, 0S) Tensile strength at 260 F. (p,s.i.) 143 12?Properties After Irradiation at 20 Watt Hours per Pound:

Relative light transmission,

19 16 30 7G 54. 5 53. 6 52.3 47. 5 sol. insol. sol. insol. Tensilestrength at 72 F. (psi). 2, 230 d, 2, 42 2, J50 Tensile strength at 260F. (p.s.i.). 04 102 138 240 Examination of the results given in Table Ishown the following:

(1) The relative light transmission of the material prepared accordingto this invention (D) was substantially unchanged by the action of theradiation, while conventionally compounded materials show a largereduction. This demonstrates the improved resistance to diseolorationdue to the action of ionizing radiation that the material of thisinvention possesses.

(2) The percent combined chlorine of the conventional material isslightly reduced by the ionizing radiation. This reduction may beattributed in a minor part to crosslinlzing, the major part of thereduction being caused by actual loss of chlorine due to the destructiveeffect of In contrast, the much greater reduction in the combinedchlorine, in the example of the material of this invention (D), isalmost wholly due to the cross'linking of the three ingredients.

(3) Further indication that cross-linking has occurred is found in thesolubility or insolubility of the material in the tetrahydrofuran(abbreviated THE above). Crosslinked materials will be found to beinsoluble in tetrahy drofuran.

(4) Tensile properties measured at room temperature show a slightdecrease for irradiated conventional compositions, and a slight increasefor irradiated compositions of this invention. Tensile propertiesmeasured at 260 F. show a striking increase after irradiation incompositions of this'invention (D above) as compared to thc conventionalmaterial (A above).

(5) Included as a part of this example are samples containingconventional plasticizer (A), a conventional plasticizer and one of thematerials used in this invention (B), and a sample containing one of thematerials of this invention alone (C). Only the sample containing thepreferred embodiment of this invention (D") shows consistent improvementin all properties, indicating that it is a combination of the threeingredients that is effective, rather than any single ingredient.

Other materials as listed in the details of this invention may besubstituted for the polyvinyl chloride, the di-2- ethylhexyl maleate, orthe styrene used above, and similar improvements in physical propertiesand in resistance to discoloration will be obtained. Also, the ratio ofester of alpha-unsaturated discarboxylic acid to styrene or the like maybe varied over the range previously given with similar improvements.

EXAMPLE 2 The specific and unique behavior of vinyl chloridevinylacetate copolymers compounded with dioctyl maleate and styrene andsubsequently subjected to ionizing radiation is shown in Example 2 wherea 4% acetate copolymer and a 13% acetate copolymcr are used. Procedureused is the same as in Example 1. Results are tabulated in Table II. Inthe case of F and H, the effect of irradiation was to transform theordinarily soft plastic composition into a hard and rigid material. Thiseffect can be shown by comparision of examples G and H, notingespecially the increase of hardness and torsional modulus afterirradiation.

Table 11 Component: (parts) E F G H Vigyite vyr r wz 6 viny c ori e 4%vinyl acetate i 100 100 Vinylite VYHH:

87% vinyl chloride. 100 100 13% vinyl acetate--- Dloetyl phthalate 50 45Dioctyl maleate. 30 28 Styrene 20 17 Properties Before Irradiation:

Relative light transmission,

percent. 50 90 Shore A hardness 60 52 79 73 Solubility in THF sol. sol.sol. sol. Torsional modulus at 72 F.. 2, 540 4, 700 Torsional hysteresisat 280 F 1 soft 1 soft soft 1 soft Tensile strength at 72 F. (p.s.i.).850 839 2, 470 2, 660 Tensile strength at 220 F.

(p.s.i.). 310 346 Tensile strength' at 260 F.

p.s.i.) 0 125 99 Properties After Irradiation at 20 Watt Hours PerPound:

Relative light transmission,

percent dark light 12 70 Shore A hardness 92 79 95 Solubility in THFmsol. sol. msol. Torsional modulus at 72 F 2, 800 49, 500 Torsionalhysteresis at 280 F 1 soft 1 brittle 1 soft 1531 Tensile strength at 72F.

(p.s.i.) 820 1,087 2, 550 3, 100 Tensile strength at 220 F.

(p.s .i.) 282 645 Tensile strength at 260 F.

(p.s.i.) 0 1 hard 101 326 1 Outside numerical range of test.

EXAMPLE 3 This example illustrates the fact that X-rays are as effectiveas high speed electrons in promoting the cross-linking of this uniquecomposition.

A composition consisting of 100 parts by weight of VYNW (a commercialvinyl chloride-vinyl acetate copolymer), 30 parts di-Z-ethylhexylmaleate and 20 parts styrene were mixed and molded into a slab 3 inchesby 5 inches by of an inch by heating in a press for 5 minutes at 350 F.The hardness of the slab was measured with a Shore D durometer and foudto be 40. it was soluble in tetrahydrofuran.

After treatment with 7.5 10 roentgens in an X-ray field the Shore Dhardness had increased to 63 and the sample was no longer soluble intetrahydrofuran. A dose of 10 roentgens corresponds approximately to theabsorption of 1 watt hour of energy per pound of material. Acorresponding treatment with 10x10 roentgens supplied by 2 million voltelectrons gave a sample with Shore D hardness of 54 which also wasinsoluble.

This application is a division of my copending application, Serial No.644,223, filed March 6, 1957, now Patent No. 3,012,001.

Having thus described my invention, what I claim and desire to protectby Letters Patent is:

1. The process of subjecting a solid state composition to high energyionizing radiation to a total cumulative radiation absorption of from 4to 40 watt-hours per pound, said solid state composition comprisingessentially (a) a vinyl chloride polymer, (b) a monomeric ester of analpha-unsaturated dicarboxylic acid with a monomeric alpohol having from1 to 18 carbon atoms, and (c) a monomeric vinylidene compound from thegroup consisting of styrene, alpha-methylstyrene, divinylbenzene,vinyltoluene, vinylchlorostyrene, dichlorostyrene, and vinylnaphthalene, said monomeric components constituting from about 20% to about50% by weight of the sum of the weights of the monomers and the vinylchloride polymer, and wherein the molal ratio of the monomericvinylidene compound to the monomeric ester is in the range of from about0.5 to about 3.0.

2. A process as in claim 1 wherein the said dicarboxylic acid isselected from the group consisting of maleic, fumaric, itaconic,citraconic, methylene malonic and mesaconic acids.

3. A process as in claim 1 wherein the said monomeric vinylidenecompound is styrene.

4. A process as in claim 2 wherein the said monomeric vinylidenecompound is styrene.

5. A process as in claim 1 wherein the said ester is an alkyl ester ofmaleic acid with an alcohol having from 1 to 18 carbon atoms.

6. A process as in claim 5 wherein the said monomeric vinylidenecompound is styrene.

7. A process as in claim 1 wherein the said vinyl chloride polymer is acopolymer of vinyl chloride and vinyl acetate containing from about 4%to about 15% vinyl acetate by weight.

8. The cross-linked product obtained by exposing a solid statecomposition to high energy ionizing radiation until from about 4 toabout 40 watt-hours per pound of energy are absorbed, said compositioncomprising essentially (a) a vinyl chloride polymer, (b) a monomericester of an alpha-unsaturated dicarboxylic acid with a monomeric alcoholhaving from 1 to 18 carbon atoms, and (c) a monomeric vinylidenecompound from the group consisting of styrene, alpha-methylstyrene,divinylbenzene, vinyltoluene, monochlorostyrene, dichlorostyrene, andvinylnaphthalene, said monomeric components constituting about 20% toabout 50% by weight of the sum of the monomers and the vinyl chloridepolymer, and wherein the molal ratio of the monomeric vinylidenecompound to the monomeric ester is in the range of from about 0.5 toabout 3.0.

9. The product of claim 8 wherein the said monomeric ester is an alkylester of maleic acid and wherein the said monomeric vinylidene compoundis styrene.

References Cited in the file of this patent UNITED STATES PATENTS2,155,590 Garvey Apr. 25, 1939 2,430,109 DAlelio Nov. 4, 1947 2,562,897Ellingboe Aug. 7, 1951 2,614,089 Harrison et al Oct. 14, 1952 2,666,025Nozacki Jan. 12, 1954 2,746,944 Naps et al. May 22, 1956 2,763,609 Lewiset a1. Sept. 18, 1956 2,843,562 Caldwell July 15, 1958 2,878,174 Raineret al Mar. 17, 1959 FOREIGN PATENTS 1,130,099 France .Tan. 30, 19571,130,100 France Jan. 30, 1957 OTHER REFERENCES Sun Modern Plastics,September 1954, pages 141-144, 146, 148, 150, 229-233, 236, 238. 1

B.N.L. 375, A.E.C., page 26 (April 1956).

Abstract of application 781,326; 656 0.6. 889; March 18, 1952,

1. THE PROCESS OF SUBJECTING A SOLID STATE COMPOSITION TO HIGH ENERGYIONIZING RADIATION TO A TOTAL CUMULATIVE RADIATION ABSORPTION OF FROM 4TO 40 WATT-HOURS PER POUND, SAID SOLID STATE COMPOSITION COMPRISINGESSENTIALLY (A) A VINYL CHLORIDE POLYMER, (B) A MONOMERIC ESTER OF ANALPHA-UNSATURATED DICARBOXYLIC ACID WITH A MONOMERIC ALCOHOL HAVING FROM1 TO 18 CARBON ATOMS, AND (C) A MONOMERIC VINYLIDENE COMPOUND FROM THEGROUP CONSISTING OF STYRENE, ALPHA-METHYLSTYRENE, DIVINYLBENZENE,VINYLTOLUENE, VINYLCHLOROSTYRENE, DICHLOROSTYRENE, AND VINYLNAPHTHALENE,SAID MONOMERIC COMPONENTS CONSTITUTING FROM ABOUT 20% TO ABOUT 50% BYWEIGHT OF THE SUM OF THE WEIGHTS OF THE MONOMERS AND THE VINYL CHLORIDEPOLYMER, AND WHEREIN THE MOLAL RATIO OF THE MONOMERIC VINYLIDENECOMPOUND TO THE MONOMERIC ESTER IS IN THE RANGE OF FROM ABOUT 0.5 TOABOUT 3.0.